1. Field of the Invention
This invention relates to a transfer substrate and a fabrication process of organic electroluminescent devices, and especially to a transfer substrate useful in the transfer of a hole-transporting material and a process for fabricating organic electroluminescent devices by using the transfer substrate.
2. Description of the Related Art
Organic electroluminescent devices making use of electroluminescence of organic materials are each formed by arranging an organic layer, which is composed of a hole transport layer and a light emitting layer stacked together, between a lower electrode and an upper electrode, and are attracting interests as light emitting devices enabling high-brightness light emission by a low-voltage DC drive.
A full-color display system making use of such organic electroluminescent devices include organic electroluminescent devices of respective red (R), green (G) and blue (B) colors formed in arrays on a substrate. In the manufacture of such a display system, light emitting layers which are formed of organic light emitting materials capable of emitting lights of the respective colors need to be formed in patterns corresponding to the respective electroluminescence devices. The formation of each light emitting layer in the corresponding pattern is performed, for example, by the shadow masking process that the light emitting material is vapor-deposited or coated through a mask formed by providing a pattern of apertures in a sheet, or by the inkjet process.
However, the formation of a pattern by the shadow masking process has difficulty in achieving further microfabrication and higher integration for organic electroluminescent devices, because further microfabrication is hardly feasible as to an aperture pattern to be formed in a mask, and due to flexing and stretching of the mask, difficulties are encountered in forming such patterned apertures at the regions of electroluminescent devices with high positional accuracy. In addition, a functional layer formed beforehand primarily of an organic layer is prone to damage by its contact with the mask in which the aperture pattern is formed, thereby causing a reduction in fabrication yield.
On the other hand, the formation of a pattern by the inkjet process can hardly realize further microfabrication and higher integration for electroluminescent devices and an enlargement for a substrate.
As a new pattern-forming process for light emitting layers made of organic materials and other organic layers, a transfer process making use of an energy source (heat source), that is, the heat transfer process has been proposed accordingly. Manufacture of a display system, which makes use of the heat transfer process, is performed, for example, as will be described next. Firstly, a lower electrode is formed beforehand on a substrate for the display system (hereinafter called “the system substrate”). On the other hand, a light emitting layer is formed beforehand on another substrate (hereinafter called “the transfer substrate”) via a photothermal conversion layer. With the light emitting layer and the lower electrode being positioned opposite each other, the system substrate and the transfer substrate are arranged. A laser beam is irradiated from the side of the transfer substrate so that the light emitting layer is thermally transferred onto the lower electrode on the system substrate. By causing the spot-irradiated laser beam to scan at this time, the light emitting layer is thermally transferred onto the lower electrode at predetermined regions with good positional accuracy (see Japanese Patent Laid-Open Nos. 2002-110350 and Hei 11-260549).
A method is also disclosed to provide organic electroluminescent devices with improved luminescence efficiency and brightness half-life upon their production by the heat transfer process. According to this method, a display substrate and a donor element are subjected to heat treatment before thermally transferring a light emitting layer (see Japanese Patent Laid-Open No. 2003-229259).